This invention relates generally to a rolling mill in which the upper and lower rolling rolls thereof are individually driven, and more particularly, to a novel differential peripheral speed rolling-type automatic plate thickness control device for such a rolling mill, in which the thickness of the plate is controlled by adjusting the difference in speed between the upper and lower rolling rolls.
In general, in a rolling mill such as a plate mill or a hot strip mill, the material thickness on the output side of the mill varies as a function of both the variation in the plastic deformation of the rolling material and the elastic deformation of the rolling mill (such as the elongation thereof). This variation in material thickness occurs even if the roll gap opening of the rolling mill is maintained at a constant value. FIG. 1 is a graphical representation of both the plastic deformation characteristic of a material and the elastic deformation characteristic of a rolling mill. In FIG. 1, curves P.sub.1 and P.sub.2 are typical plastic deformation curves of rolling material, and curves M.sub.1 and M.sub.2 are typical rolling mill elastic deformation curves.
The plastic deformation characteristic of a rolling material depends upon the input material thickness H, the output material thickness h, an average deformation resistance k and a material plate width W, or EQU F=f(H,h,k,W) (1)
In FIG. 1, this relationship is shown by curves M and M.sub.2. Thus, the input plate thickness is H.sub.1, the plastic curve is P.sub.1 and the rolling mill elastic curve is M.sub.1. If these values are held constant, and the roll gap opening is S.sub.1, then the rolling load is F.sub.1 and the output plate thickness is h.sub.1 (defining the operating point (1)).
If, at a time instant 2 until which the rolling has been advanced, the input side plate thickness is changed to H.sub.2 (H.sub.1 &lt;H.sub.2) and the other variables are maintained constant, the plastic curve changes from P.sub.1 to P.sub.2. As a result, the rolling load increases to F.sub.2 (F.sub.1 &lt;F.sub.2) and the output material thickness increases to h.sub.2 with the elongation of the rolling mill (defining the operating point (2)).
As is apparent from the above description, if the variation in the plastic characteristic of a rolling material is left uncontrolled, it is impossible to produce series of plates of uniform thickness. For manufacturing reasons, it is necessary to employ means for making the output material thickness constant. Heretofore, an Automatic Gauge Control proposed by British Iron & Steel Research Assn. (BISRA AGC) has been employed for controlling the output plate thickness. The BISR AGC is a method of correcting the roll opening so that the elongation of the rolling mill due to a variation in rolling load is cancelled out. The operating principle of the BISRA AGC is as follows:
If the elastic characteristic of a rolling mill can be approximated by a straight line, and the inclination angle of the straight line (hereinafter referred to as "a mill constant", when applicable) is represented by M, then the rolling mill output plate thickness h can be expresed by the equation: EQU h=S+F/M (2)
where h is the material thickness (mm) at the output of the rolling mill, S is the initial roll gap degree (mm), F is the rolling load (ton), and M is the mill constant (ton/mm).
From equation (2), the variation of the output side plate thickness can be expressed as: EQU .DELTA.h=.DELTA.S+.DELTA.F/M (3)
Accordingly, the variation in rolled thicknesses can be reduced by correcting the roll opening degree: EQU .DELTA.S=-.DELTA.F/M (4)
FIG. 2 is a block diagram showing a conventional BISRA AGC. In FIG. 2, reference numeral 1 designates the work rolls of a rolling mill which is supported by the back-up rolls 2. A depressing screw 3 imparts a compressive force on both back-up rolls 2 and work rolls 1. The screw 3 is threadingly engaged to the rolling mill housing 4. A depressing motor 5 adjusts the roll opening degree by turning screw 3. A roll opening degree automatic positioning device (hereinafter referred to as "an APC device"). A roll opening degree detector 7 and a load cell 8 detect the roll opening degree and the rolling load, respectively. A memory device 9 and an arithmetic block 10 for calculating elongations of the rolling mill receive input signals from load cell 8. Finally, 11 denotes a tuning factor setting device, and S denotes a material under rolling.
The operation of the above-described circuitry will now be described. When the material S is fed through the rolling mill housing 4, the instantaneous rolling load Fo is stored in the memory device 9, and the BISRA AGC is initiated. As the work material is advanced through housing 4, the variations in rolling load F are detected as a function of the stored value Fo, and equation (4) is calculated in the elongation calculating block 10. The output of the calculating block 10 is applied (through tuning factor device 11) as a command value to the APC device 6.
As a result, the rolling mill roll opening degree is corrected as a function of operating point (3) in FIG. 1. The tuning factor (11) in FIG. 2 is a constant which determines the degree to which the elongation of the rolling mill is corrected. The tuning factor is set in a range of 0.ltoreq..alpha..ltoreq.1, where .alpha.=1 means that the elongation is corrected 100% and .alpha.=0 means that the AGC is not operated.
The conventional BISRA AGC, designed as described above, suffers from a drawback in that the operation of the AGC may accelerate the rolling load variation. Referring to FIG. 1, the rolling load variation .DELTA.F.sub.2 =F.sub.2 -F.sub.1 when the AGC is not operated, and when the AGC is operated, the rolling load variation .DELTA.F.sub.3 =F.sub.3 -F.sub.1, such that .vertline..DELTA.F.sub.2 .vertline.&lt;.vertline..DELTA.F.sub.3 .vertline. (i.e., the change in force is enhanced during AGC operations). Further, as the rolling load varies, the deflection of the rolling rolls varies, as a result of the flatness of the product is varied; that is, the quality (in the direction of plate width) of the product is degraded. Accordingly, in a conventional hot strip mill, it is often impossible to apply the BISRA AGC of the prior art to thin and wide strips. Also, in the case of a conventional thick plate mill, it is occasionally necessary to add a special pass under low pressure called a "shape correcting pass" after the final AGC pass.
The ratio of (a) the rolling load variation .DELTA.F.sub.3 at the BISRA AGC (with the tuning factor .alpha. being equal to (1) to (b) the rolling load variation .DELTA.F.sub.2 provided when the AGC is not operated, can be expressed as: ##EQU2## where, M is the mill constant (ton/mm), and Q is the elastic constant (ton/mm), i.e., the inclination of the plastic curve near the operating point.
Thus, in the case of an ordinary hot strip mill final stand, wtih a material having a strip width of 1500 mm and a thickness of 1.6 mm, and where Q=3000 tons/mm and M=600 tons/mm approximately, the ratio .DELTA.F.sub.3 /.DELTA.F.sub.2 .perspectiveto.6. When the AGC is operated with .alpha.=1 under the above-described conditions, the rolling load variation is about 300 tons at the skid mark portion (i.e., where the wavy edges are formed).
Another drawback of the conventional BISRA AGC is as follows: normally, the BISRA AGC should have a mill (elastic) constant as a "model" for the calculation of mill elongation (as is apparent from FIG. 2). However, since the mill constant M is dependent on such factors as material width, plate thickness, roll diameter and rolling force, the accuracy of the estimated mill constant is limited, and accordingly, the improvement of the accuracy of AGC is also limited.